Before you take it back again, go thru your stash, take two pieces of fabric and some batting and tell the service tech your not leaving until you try it out. I would plug it in and run it for at least 15 minutes the way you would do it at home. I wouldn't be making a third trip. Also, start checking now for someone to service your machine for the next time. Be prepared. That's the last time I'd go there for sure.